Resonant power circuits, such as lamp ballasts, commonly use frequency as the main control variable for maintaining output current or voltage constant or, in the case of dimming circuits, for changing output current or voltage. For some high power factor ballast circuits, e.g., well-known valley fill circuits, such as of a type described in U.S. Pat. No. 5,012,161 of J. C. Borowiec and S-A El-Hamamsy, issued Apr. 30, 1991, the current in the lamp is modulated by the bus voltage. In order to meet the specifications for lamp life, the lamp current crest factor has to be less than about 1.7. The lamp current crest factor (ccf) is defined as the ratio of peak current to rms current; an unmodulated sine wave has a crest factor of 1.414. A simple, economical circuit that controls the lamp current is therefore needed in order to meet lamp life specifications.
Specialized integrated circuits (IC's) use voltage-controlled oscillators (VCO's) for generating variable frequency drive signals. Disadvantageously, however, such circuits are typically too expensive to use in very low-cost applications, such as, for example, compact fluorescent lamp ballasts.
Accordingly, it is desirable to provide a variable frequency generator for use in the feedback loop of resonant power circuits, such as, for example, lamp ballasts or other applications in which either the current or voltage of a resonant switching converter is controlled by varying the switching frequency such as, for example, off-line power supplies and dc-to-dc converters. It is also desirable to provide a fixed duty ratio, variable frequency, current-controlled or voltage-controlled square wave gate drive in an IC with a minimal number of circuit components. Such a controller should have a high gain so that error in the output is minimized.